Generalized subproc_execute and added tests

[pryce-turner]

Why are the changes needed?

subproc_execute was too restrictive, this allows passing of arbitrary kwargs with sensible defaults

What changes were proposed in this pull request?

modified subproc_execute and 2 tests for shell and non-shell exec

How was this patch tested?

2 unit tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e107994) 85.71% compared to head (a667728) 85.73%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2072      +/-   ##
==========================================
+ Coverage   85.71%   85.73%   +0.01%     
==========================================
  Files         313      313              
  Lines       23396    23398       +2     
  Branches     3501     3501              
==========================================
+ Hits        20055    20061       +6     
+ Misses       2733     2730       -3     
+ Partials      608      607       -1     

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